Great question! Below are the definitions of a manipulated, responding, and the constant (controlled) variables:

MANIPULATED VARIABLES: These are variables that you change on purpose in your experiment. It is what you are testing.

RESPONDING VARIABLES: These are the changes that happen in your experiment because of what you changed.

CONSTANT VARIABLES: These are the variables that are held constant or are assumed to stay the same during the experiment.

Need an example? Let’s use our seed germination experiment from our last check in. The manipulated variable would be the temperature. We wanted to change the temperature so that we could test how it effected seed germination. The responding variable would be how fast the seeds germinated. We changed the temperature, so some of the seeds should germinate quicker, some slower, etc. Finally, the constant variables are what we would keep the same, like the amount of soil, when and how much we water them, etc. Choosing which variables to control is important. You want to make sure that you limit what you change, so that you can control your results. For example, if we are not consistent in the amount we water each seed, then we won’t know if it was the temperature that had effected seed germination or the amount of water. I hope this helps!

When you conduct a Science Fair experiment, you should be conducting multiple trials of your experiment, to test for overall accuracy. The constant variable is what you keep the same for each trial. Think of our lightbulb experiment last week. We conducted multiple trials to test whether or not we had built a complete circuit. The manipulated variable was what we changed each time: the position of the lightbulb. The responding variable was the result we had each time: the lightbulb either lit up or it didn’t light up, we had a complete circuit or we didn’t. The controlled variable was what was the same each time: the wire, the battery, and the lightbulb we used never changed. These things were our controls.